SilcoTek enhanced the aluminum CVD coating process.  The results are impressive!

Improved Inertness and Corrosion Resistance for Aluminum

SilcoTek® refined the coating process to expand our already impressive coating performance to aluminum.  In this blog we discuss how we refined our process and learn how our process, called SP12, compares to our standard process on aluminum substrates.  

How do we coat aluminum?

Aluminum can be a difficult substrate for silicon to form an effective bond to.  The resulting coating can have at times a flat dark or blotchy appearance, not the customary bright rainbow appearance expected from our coating (as seen in the top photo of our blog post).  More concerning, the performance of the coated surface may, for some aluminum substrates, be less effective when compared to our standard coating. 

The R&D Team set out to study and understand the silicon-aluminum interface and to refine the coating process to match the performance of our standard coating process. 

Chemical Vapor Deposition Benefits and Limitations

After extensive research, the R&D Team found that the aluminum substrate had a catalytic effect at the silicon-aluminum interface that promoted silicon nanowire growth.  The Team then set out to develop a "primer" layer that minimized the catalytic effect.  The characteristics of the resulting primer process include: 

• Use a primer layer to isolate aluminum from the a-Si coating.
• Primer layer itself is immune to aluminum’s catalytical effect.
• Primer layer bonds well to both substrate and a-Si coating.
• Primer layer can be applied in the same CVD process to be production friendly.

SilcoTek® developed a proprietary process that meets the above requirements and can be implemented within our current process capabilities.  The graphic below demonstrates how the primer layer acts as a barrier between the aluminum substrate and the silicon coating but enhances the performance of the CVD process when applied to aluminum substrates.

Standard Coating                      Aluminum Coating (SP12)

The results of the SP12 coating process showed comparable coating appearance to standard coated stainless steel.  To verify the coating performance, the team tested uncoated aluminum, standard coated aluminum, and coated aluminum using the SP12 process.  The team tested sample coupons for corrosion resistance and inertness.

Corrosion Performance

Uncoated aluminum test coupons, standard coating process coupons, and SP12 process coupons were immersed in 5% hydrochloric acid solution (HCl) for 20 minutes.  The photo below shows intense reaction in the solution while the SP12 coated sample (lower left) does not react in the solution. 

Tests done in 5% HCl solution at room temperature, using 6061 Aluminum coupons

                       Uncoated Aluminum Sample     Standard Coating       SP12 Coated Aluminum

Salt Fog Testing

12 week salt fog testing on uncoated, standard coated, and SP12 coated samples show marked differences in corrosion resistance.  The test was carried out in a salt fog (NaCl) chamber per ASTM G85.

Sample photographs (top image shown below), and enhanced magnification images (below middle) show significant corrosion while the coated part using the SP12 process show minimal corrosion. 

This is supported by energy dispersive spectroscopy (EDS) analysis (lower tables).  The higher oxygen content in EDS (highlighted) indicates more corrosion.  That's because aluminum corrodes by oxidation to form aluminum oxide.  The SP12 EDS oxygen results indicate O₂ levels are significantly lower than the standard coating process or uncoated aluminum (8.8% vs. 54.1% and 63.9% respectively).

Want to learn more about how SilcoTek can improve the corrosion resistance of your products?  Get lots of corrosion performance data and see how our coatings improve corrosion resistance without using costly super alloys, or part redesign.  Click the box below to read our latest corrosion presentation.

Inertness Testing

In addition to corrosion resistance, customers may also need an inert aluminum surface.  An inert coated aluminum surface will save significant weight when compared to stainless steel coated products which will benefit field testing or testing that requires sample transport.  

Liner testing done by a customer showed comparable performance of SP12 applied SilcoNert 2000 to standard process SilcoNert 2000-coated borosilicate liners in trace level pesticide analysis.  The test criteria is highlighted below:  

• Test conditions: Endrin at 50 pg/µL and DDT at 100 pg/µL.
• Typical pass/fail criteria are below 10% breakdown rates, which are met by both coatings.

Test breakdown percentage graphs (below) for the SP12 process were comparable if not better than the standard application process for SilcoNert 2000.  

Additionally SP12-coated aluminum has shown good inertness properties in sulfur and ammonia analysis in customer applications.  Inertness results are comparable to standard process-coated stainless steel counterparts.

Mercury Testing

Mercury inertness testing was performed by Ohio Lumex under a mercury concentration of 9.8 ng/L, injected into the 10.6 mL internal volume of each cell.

Decay curves below show comparable performances between SP12 and standard SilcoNert 2000 processes.

The elapsed time for decay below 10% (table below), shows that the SP12 coated SilcoNert samples performed about 16% better than the standard SilcoNert 2000 process.  Both coating processes performed orders of magnitude better than the uncoated surface.

Summary

Aluminum and its alloys present challenges for silicon CVD deposition.  Testing shows the SP12 process offers the same great benefits of our standard coating process without the negative effects when coating aluminum. 

The issues our new SP12 process address include:

• Same process that works well on SS does not work for aluminum.
• Aluminum catalyzes silicon crystallization and promotes nanowire growth.
• Poor cosmetic output and coating performances as a result.

A proprietary process (SP12) was developed by SilcoTek® to address aluminum substrate issues.  The process employs a strategy of applying a primer layer to isolate the aluminum substrate from the silicon coating while maintaining the same external surface chemistry and coating performance, including improved corrosion resistance and superior inertness.

Performance benefits of SP12:

• Consistent and reliable output
• Enabled the same excellent inertness properties known to SS on aluminum
• Greatly improved corrosion protection on aluminum substrate

A few items to note about the SP12 process:

• 5000-series Al alloys present a special compatibility risk.  As a safety precaution, SilcoTek still does not treat 5000-series aluminum alloys.

• SilcoTek can coat most grades of aluminum alloys.  It is important that customers inform SilcoTek of the exact material grade of aluminum substrates prior to sending parts.  Learn more by clicking here. 

• The SP12 process involves exposure of the aluminum substrate to elevated temperature.  Aluminum is weakened at temperatures in excess of 100° C.  Because of the temperature related strength limitations associated with aluminum; SilcoTek does not recommend coating cylinders or any other aluminum product that will be pressurized or used in high stress applications.  Learn more by clicking here. 

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